This invention relates to the preparation of 3-(methylthio)propanal, and more particularly to a continuous process for the direct manufacture of 3-(methylthio)propanal in a gas/liquid reaction system.
3-(Methylthio)propanal (hereinafter xe2x80x9cMMPxe2x80x9d) is an intermediate for the manufacture of both d,l-methionine and 2-hydroxy-4-(methylthio)butanoic acid (xe2x80x9cHMBAxe2x80x9d). Methionine is an essential amino acid in which components of the animal feed compositions are commonly deficient. HMBA provides a source of methionine, and is widely used as a methionine supplement in animal feed formulations. MMP relatively free of impurities is typically required for the manufacture of HMBA or methionine.
MMP is produced by reaction of acrolein with methyl mercaptan. In a conventional process for the preparation of MMP, liquid acrolein and methyl mercaptan are introduced into a reactor containing liquid phase MMP product. Reaction takes place in the liquid phase. In order to produce MMP of desired quality, refined acrolein is used in the process, and/or the MMP product is distilled before use in the manufacture of either HMBA or methionine.
Acrolein is a highly toxic and flammable material. It is conventionally prepared by vapor phase oxidation of propylene over a solid phase catalyst, producing a crude gaseous reaction product which contains water vapor, acrylic acid, acetaldehyde, and other organic by-products. Typically, the gas is treated to remove acrylic acid, then contacted with refrigerated water for absorption of acrolein. The resultant aqueous solution is distilled to recover the absorbed acrolein and other organic components. The crude acrolein is then refined to reject lower boiling impurities such as acetaldehyde, producing a purified liquid acrolein product. The refined liquid acrolein is stored for use in the manufacture of MMP.
Storage of liquid acrolein involves significant toxicity, fire and explosion hazards. High capital and operating costs are consequently incurred in providing for the safe handling of this material. The cost of handling acrolein could be substantially reduced if gas phase acrolein were transferred directly and continuously from the acrolein manufacturing process to the MMP reactor without storage or condensation. However, since the conventional commercial processes for the preparation of MMP involve liquid phase reactions, the need to condense the gaseous acrolein product has been considered unavoidable. Moreover, because the conventional process typically uses a batch reaction system, condensation and in-process storage of liquid acrolein is necessary as a surge buffer between operation of the acrolein process and the MMP reactor.
Netherlands patent No. 6809647 describes a process in which acrolein is produced by catalytic oxidation of propylene and the acrolein-containing reaction gas mixture is passed to a vertical reaction column in which MMP is formed. MMP is circulated through the reaction column and both the acrolein-containing gas and methyl mercaptan are added near the bottom. MMP exiting the column contains a separate aqueous phase which is removed in a separator. MMP from the separator is partially recycled to the reaction column. A sodium bicarbonate solution is supplied to the circulating MMP. MMP product removed from the circulating reaction system is distilled at a pressure of 100 mmHg.
U.S. Pat. No. 4,225,516 describes a continuous process for the manufacture of MMP from the acrolein product gas obtained in the catalytic oxidation of propylene. In this process, the gas is first treated for removal of acrylic acid, then cooled to condense water vapor. To reduce the water vapor content to a level acceptable in the MMP reaction, the final condensation temperature is 0xc2x0 to xe2x88x925xc2x0 C. The treated and cooled acrolein gas stream is contacted with a stream of liquid MMP in a countercurrent absorption tower, resulting in absorption of acrolein in the MMP. The MMP liquid stream containing dissolved acrolein is circulated to an MMP reactor where methyl mercaptan is added. The process proceeds by reaction of methyl mercaptan with MMP to form the hemimercaptal of MMP, and the hemimercaptal in turn reacts with acrolein in the liquid phase to produce additional MMP. Thus, the process requires the presence of up to 1% by weight of the hemimercaptal in the reaction mixture. MMP product is withdrawn from the system at a rate equivalent to MMP production in the reactor, while the bulk of the MMP stream is recirculated to the acrolein absorber.
To provide for quantitative absorption of acrolein in MMP, the ""516 patent requires cooling the circulating MMP to a temperature 0xc2x0 to xe2x88x9215xc2x0 C. before it enters the absorber. The refrigeration required for condensing water vapor at 0xc2x0 to xe2x88x925xc2x0 C. and cooling MMP to as low as xe2x88x9215xc2x0 C. contributes substantially to the capital and operating expense of the ""516 patent process. Moreover, because the reaction proceeds through formation of the hemimercaptal, the kinetics of the conversion reaction are relatively slow, resulting in less than desirable productivity and thus further adding to the cost of operation of the process.
Although sub-zero absorption increases acrolein recovery at equilibrium, it also increases the absorption of impurities, such as acetaldehyde, in the MMP product. Moreover, since the scrubber is separate from the reactor, acrolein absorbed in the scrubber is not consumed immediately in the absorption zone. As a consequence, acrolein tends to accumulate in the liquid phase, which decreases the driving force for mass transfer. The high concentration of acrolein in MMP liquid also increases the possibility of forming by-products from reactions between acrolein and MMP.
Among the several objects of the present invention are the provision of an improved process for the preparation of MMP; the provision of such a process which can be operated in a continuous mode; the provision of such a process which can be operated with high productivity; the provision of such a process which can be operated with a relatively crude acrolein raw material; the provision of such a process which does not require refrigeration for absorption or condensation of acrolein; the provision of such a process which eliminates the need for storage of liquid acrolein, in particular, the provision of such a process which can be operated using a gaseous acrolein feed obtained directly from the continuous oxidation of propylene or other suitable hydrocarbon; the provision of such a process which can be operated without formation of a separate aqueous phase in the MMP reaction mixture; and the provision of such a process which can produce high quality MMP for direct use in the preparation of methionine or HMBA without the need for further purification.
Briefly, the invention is directed to a process for the continuous preparation of MMP in which a liquid reaction medium is contacted with a gaseous acrolein feed stream in a gas/liquid contact zone. The reaction medium contains MMP, methyl mercaptan and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor and non-condensable gas. The relative proportions of acrolein and methyl mercaptan entering the contact zone are substantially stoichiometrically equivalent. Acrolein is transferred from the feed stream to the reaction medium and reacts directly with methyl mercaptan in the medium, without substantial formation of the intermediate hemi(methylthio)acetal of MMP, to produce a liquid reaction product containing MMP. Non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone. Acrolein and methyl mercaptan react in the liquid medium in a reaction zone that comprises the gas/liquid contact zone and a circulation zone into which the liquid reaction product is discharged from the gas/liquid contact zone and through which the circulating fraction is circulated back to the gas/liquid contact zone. Methyl mercaptan is introduced into the reaction zone at a location or locations such that no excess of methyl mercaptan prevails in any region of the reaction zone for time long enough for substantial formation of the intermediate hemi(methylthio)acetal.
The invention is further directed to a process for the continuous preparation of MMP in which a liquid reaction medium is contacted with gaseous acrolein feed stream in a gas/liquid contact zone. The reaction medium contains MMP, methyl mercaptan, and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor, non-condensable gas, and water vapor. Acrolein is transferred from the feed stream to the reaction medium and reacts with methyl mercaptan in the medium to produce a liquid reaction product containing MMP. The ratio of water vapor to acrolein in the acrolein feed stream is such that no substantial second liquid phase is present in the liquid reaction product as a result of condensation of water in the feed stream. Non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone.
The invention is also directed to a process for the continuous preparation of MMP in which a liquid reaction product is contacted with a gaseous acrolein feed stream in a gas/liquid contact zone, the reaction medium containing MMP, methyl mercaptan and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor, non-condensable gas, and water vapor. Acrolein is transferred from the feed stream to the reaction medium and reacts with methyl mercaptan in the medium to produce the liquid reaction product containing MMP. The molar ratio of water vapor to acrolein in the acrolein feed stream is not greater than about 0.3. The non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone.
Also contemplated by the invention is a process for the continuous preparation of MMP in which a liquid reaction medium is contacted with the gaseous acrolein feed stream in a gas/liquid contact zone through which the feed stream and the reaction medium are passed countercurrently. The reaction medium contains MMP, methyl mercaptan, and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor and non-condensable gas, whereby acrolein is transferred from the feed stream to the reaction medium and reacts with methyl mercaptan in the medium to produce a liquid reaction product containing MMP. The liquid hold-up in the countercurrent gas/liquid contact zone is sufficient to effect conversion in the gas/liquid contact zone of at least 90% of the acrolein contained in the feed gas. The non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction and the circulating fraction is recycled to the gas/liquid contact zone.
The invention is also directed to a process for the continuous preparation of MMP in which a liquid reaction medium is contacted with a gaseous acrolein feed stream in a gas/liquid contact zone, the reaction medium containing MMP, methyl mercaptan and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor and non-condensable gas. Acrolein is transferred from the feed stream to the reaction medium and acrolein and methyl mercaptan are reacted in the reaction medium in a first reaction zone comprising the gas/liquid contact zone, producing an intermediate liquid reaction product. The non-condensable gas is separated from the intermediate liquid reaction product, the intermediate liquid reaction product is divided into an intermediate product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone. The first reaction zone comprises the gas/liquid contact zone and a circulation zone into which the liquid reaction product is discharged from the gas/liquid contact zone and through which the circulating fraction is circulated back to the gas/liquid contact zone. The intermediate product fraction is passed through a plug flow reactor to convert residual acrolein and methyl mercaptan to MMP.
Further contemplated by the invention is a process for the continuous preparation of MMP in which a liquid reaction product is contacted with the gaseous acrolein feed stream in a gas/liquid contact zone. The reaction medium contains MMP, methyl mercaptan, and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor, non-condensable gas and acrylic acid vapor. Acrolein is transferred from the feed stream to the liquid reaction medium and reacts with methyl mercaptan in the medium to produce a liquid reaction product containing MMP. The molar ratio of acrylic acid vapor to acrolein in the acrolein feed stream is not greater than about 0.1. Non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone.
The invention is also directed to a process for the continuous preparation of MMP in which acrolein vapor is produced by a vapor phase catalytic oxidation of a hydrocarbon to produce a crude acrolein reaction product stream. The crude acrolein reaction product stream is cooled to condense water vapor and acrylic acid therefrom and produce a cooled acrolein gas stream for conversion to MMP, the feed stream comprising acrolein and non-condensable gas. A liquid reaction medium is contacted with a gaseous acrolein feed stream comprising said cooled acrolein gas stream in a gas/liquid contact zone in which the total pressure is not greater than about 3 atmospheres. The reaction medium contains MMP, methyl mercaptan and a catalyst for the reaction between methyl mercaptan and acrolein. Acrolein is transferred from the feed stream to the reaction medium and reacts with methyl mercaptan in the medium to produce a liquid reaction product containing MMP. Non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone.
The invention is further directed to a process for the continuous preparation of MMP in which a crude reaction product gas stream obtained from the catalytic oxidation of a hydrocarbon is cooled, thereby producing a cooled gas stream comprising acrolein and a condensate comprising water, acrylic acid and a residual proportion of acrolein. The condensate is separated from the cooled gas stream and the condensate is fractionally distilled to produce an overhead fraction comprising acrolein and a bottoms fraction which is substantially free of acrolein. The overhead fraction is mixed with the cooled gas stream to produce a combined acrolein stream. A liquid reaction medium is contacted with a gaseous acrolein feed stream in a gas/liquid contact zone. The reaction medium contains MMP, methyl mercaptan and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises the combined acrolein gas stream and contains acrolein, non-condensable gas, and water vapor. Acrolein is transferred from the feed stream to the reaction medium and reacts with methyl mercaptan in the medium to produce a liquid reaction product containing MMP. Non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone.
The present invention is also directed to a process for the continuous preparation of 3-(methylthio)propanal. The process comprises contacting a liquid reaction medium with a gaseous acrolein feed stream in a gas/liquid contact zone of a reaction zone. The reaction medium contains 3-(methylthio)propanal, methyl mercaptan, and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor and noncondensable gas. The gaseous acrolein feed stream and the reaction medium are caused to flow co-currently through the gas/liquid zone. Acrolein is transferred from the feed stream to the reaction medium and reacts with methyl mercaptan in the medium to produce a liquid reaction product containing MMP. Noncondensable gas is separated from the liquid reaction product and the reaction product is divided into a product fraction and a circulating fraction. The circulating fraction is recycled to the gas/liquid contact zone. The heat of reaction is removed from the reaction zone by indirect transfer of heat from said liquid reaction medium to another fluid. The rate of circulation of the liquid medium and the location from which heat is removed from the reaction zone are such that the temperature of the liquid reaction medium does not vary more than about xc2x15xc2x0 F. throughout said reaction zone.
The present invention is further directed to a process for the continuous preparation of 3-(methylthio)propanal in which a gaseous acrolein feed stream is contacted with a liquid reaction medium, the reaction medium containing 3-(methylthio)propanal and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor and non-condensable gas, whereby acrolein from the feed stream is transferred to the liquid medium. Methyl mercaptan is also introduced into the reaction medium. Acrolein is reacted with methyl mercaptan in the medium in a first reaction zone that comprises the gas/liquid contact zone to produce a liquid reaction product containing 3-(methylthio)propanal, the liquid reaction medium exiting the gas/liquid contact zone containing not more than about 5% by weight acrolein. The reaction product from the first reaction zone is passed through a second reaction zone to convert residual acrolein and methyl mercaptan to 3-(methylthio)propanal.
The invention is further directed to a process for the continuous preparation of 3-(methylthio)propanal in which a liquid reaction medium is contacted with the gaseous acrolein feed stream and a gas/liquid contact zone through which the feed stream and the reaction medium are passed countercurrently. The reaction medium contains 3-(methylthio)propanal and a catalyst for reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor and non-condensable gas whereby acrolein is transferred from the feed stream to the liquid reaction medium, the liquid medium exiting the gas/liquid contact zone containing not more than about 5% by weight acrolein. Methyl mercaptan is also introduced into the liquid reaction medium and acrolein and methyl mercaptan are reacted in such medium to produce a liquid reaction product containing 3-(methylthio)propanal.
Further contemplated by the invention is a process for the continuous preparation of 3-(methylthio)propanal in which a gaseous acrolein feed stream is contacted with a liquid reaction medium, the reaction medium containing 3-(methylthio)propanal and a catalyst for the reaction between methyl mercaptan and acrolein, the gaseous acrolein feed stream comprising acrolein vapor and non-condensable gas, whereby acrolein from the feed stream is transferred to the liquid medium. Methyl mercaptan is also introduced into the reaction medium and acrolein is reacted with methyl mercaptan in the medium in a first reaction zone comprising the gas/liquid contact zone to produce a liquid reaction product containing 3-(methylthio)propanal. The liquid reaction product exiting the gas/liquid contact zone contains between about 0.8% and about 5% by weight acrolein. The reaction product from the first reaction zone is passed through a second reaction zone to convert residual acrolein and methyl mercaptan to 3-(methylthio)propanal.
The invention is further directed to a process for the continuous preparation of 3-(methylthio)propanal comprising contacting the liquid reaction medium with a gaseous acrolein feed stream in a gas/liquid contact zone through which the feed stream and the reaction medium are passed countercurrently, the reaction medium containing 3-(methylthio)propanal in a catalyst for the reaction between methyl mercaptan and acrolein, the gaseous acrolein feed stream comprising acrolein, vapor, and non-condensable gas, whereby acrolein is transferred from the feed stream to the reaction medium. A primary methyl mercaptan feed stream is introduced into the countercurrent gas/liquid contact zone at a rate which provides a supply of methyl mercaptan that is less than stoichiometrically equivalent to the rate of introduction of acrolein into the contact zone. Acrolein is reacted with methyl mercaptan in a first reaction zone in which acrolein is in stoichiometric excess with respect to methyl mercaptan. The reaction medium exiting the first reaction zone is divided into an intermediate product fraction and a circulating fraction. The circulating fraction is recirculated to the countercurrent gas/liquid contact zone and a secondary methyl mercaptan feed stream is introduced into the intermediate product fraction. The intermediate product fraction is introduced into a second reaction zone where methyl mercaptan reacts to convert residual acrolein in the intermediate fraction to 3-(methylthio)propanal. In an alternative to this embodiment of the process, an intermediate reaction product exits the first reaction zone, and a secondary methyl mercaptan feed stream is introduced into the intermediate reaction product outside the countercurrent gas/liquid contact zone. The intermediate reaction product is passed into a second reaction zone where methyl mercaptan introduced in the secondary feed stream reacts with residual acrolein in the intermediate reaction product to produce MMP. The reaction medium exiting the second reaction zone is divided into a product fraction and a circulating fraction; and the circulating fraction is recirculated to the countercurrent gas/liquid contact zone.
The invention is further directed to the process for the continuous preparation of 3-(methylthio)propanal in which a liquid reaction medium is contacted with a gaseous acrolein feed stream and a gas/liquid contact zone through which the feed steam and the reaction medium are passed countercurrently. The reaction medium contains 3-(methylthio)propanal and a catalyst for the reaction between methylmercaptan and acrolein, the gaseous acrolein feed stream comprising acrolein, vapor, and non-condensable gas, whereby acrolein is transferred from the feed stream to the reaction medium. The liquid medium exiting the gas/liquid contact zone contains between about 0.8% and about 5% by weight acrolein. Methyl mercaptan is introduced into the reaction medium and acrolein is reacted with methyl mercaptan in the medium to produce a liquid reaction product containing 3-(methylthio)propanal. Preferably, reaction of acrolein with methyl mercaptan in the reaction medium occurs within the gas/liquid contact zone to produce a reaction product leaving the gas/liquid contact zone with an acrolein content of between about 0.8% and about 5% by weight.
Other objects and features will be in part apparent and in part pointed out hereinafter.